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Spirals in Energy Kite Systems

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Dan Parker      
Patent number: 6948910
Spiral-based axial flow devices by Ronald R. Polacsek
Filed: Jul 12, 2002                Issued: Sep 27, 2005
September 21, 2009

Several AWE concepts rely on long torque-tubes to bring rotary energy to the ground, but there are severe scaling limitations. A large torque-tube that reliably resists local buckling structural failure is far too heavy to fly well.

But water is another story- a roughly 1000 times denser medium than air. A massive torque tube can easily be neutrally buoyant & work well in moving water at high megawatt scales. Cheap steel or ferrocement construction would suffice.

One of Doug Doug Selsam's AWE schemes puts high aspect ratio turbine blades along a long torque-tube. There are the standard concerns about the liability of high-speed blades whipping around people and property while creating a bird- or fish- "blender."  But low aspect ratio turbines (Low AR) like Dan Parker's ease this concern. Yes, Low AR needs a step-up gear to match Hi AR rpm, but at common slow water current speeds (single-digit knots) self-start sooner--and still extract similar power.

Last week I towed a "spiral airfoil" in water, a common kite spin-tail (Black Lake, Illwaco, WA). The novelty item performed much as in air, but slower, an old fluid-dynamicist's trick based on dynamic similarity. The spin-tail developed lots of pull & torque at a knot or two of tow speed. As speed increased it gracefully furled into a longer narrower screw geometry that slowed drag increase while maintaining a fairly level torque/rpm output. Sweeeet. I set another spin-tail in the small stream behind KiteLab & it spun for hours without a problem in the small shallow channel that had skunked my High AR hydrofoil experiments.

The spin-tail laid flat on its axis reveals itself as a basic logarithmic spiral cut from a single piece of untailored fabric. A plastic whisker is sewn into its outer (leading) edge to stiffen it. Is there another fluidic device so elegant? Though ancient examples abound, the oldest extant description of the famous log spiral is by Descartes. Jacob Bernoulli went so wild over the form proclaiming it the "spira mirabilis".

When the logarithmic spiral is constrained within a circle it produces a beautiful curved foil plan that initially broadens from the center then tapers to a point at the circle's edge. It naturally develops helical pitch when tensioned along its axis. It also is reversible; just flip it over & it changes handedness.

When you put a conventional turbine or log spiral (or two or more) at the end of a torsion tube you can bring the torque aboard a boat & keep the generator out of the water; a big win. Call it a "swizzle-stick" turbine. In '07 I played with a tippin' turbine generator in the Chinook River tidal flow using slow-flight RC propeller.

Two other spirals are worth mentioning. The Archimedean spiral is cruder & has greater induced drag as a spiral foil. The simple helical spiral has a constant diameter curl & finds use in bolt threads & as a strake wound around smoke stacks to break up destructive bluff body oscillation in wind.

A sufficiently long helical strake along a torque tube set axial to a flow has an amazing ability to exceed the Betz Limit, which is based on a disc assumption. It can far surpass the ~60% extraction limit imposed on a short axis turbine of a given disc area by progressively entraining more surrounding flow downstream. Envision long rolling snakes in a water channel developing tremendous torque without harming aquatic life. In open water, traction kites might tow these fish-friendly hydropower turbines.

A future note will more fully consider the uses of short "wings" like strakes, chines, edges, oblique bluff bodies (esp. kite line), and many kite wings. A short "ugly" wing can have reasonable L/D at low Re & the high drag at high Reynolds number (Re) is useful for progressive stability mechanisms. Wing-loading will be explained as a simple general predictor of AWE reliability.              ~ Dave Santos

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